Hurricane Isaac Impacts on the Pontchartrain Basin

Hurricane Isaac:
Summary of Impacts on the Pontchartrain Basin
Ezra Boyd. PhD
Lake Pontchartrain Basin Foundation
Coastal Sustainability Program
November 12, 2012
I) Introduction and Description of Storm
Hurricane Isaac was a slow moving Category 1 that impacted the central Gulf Coast, including
the Pontchartrain Basin, on August 2012. While tides slowly begun rising earlier that day,
noticeable storm impacts first reached the region on the evening of Tuesday, August 28, with
heavy rain and strong winds through that night and the entire next day. This storm followed
what many experts considered the worst possible path for greater New Orleans (GNO), slowly
skirting the southeast Louisiana coast in a westward direction. During the early hours of that
Wednesday morning, the storm stalled for a few hours with the eye due south of New Orleans
and just offshore of Grand Isle, La.
This track meant that Southeast Louisiana endured 48+ hours of strong, easterly winds. The
persistence of winds contributed to a stronger storm surge in tidal waterbodies, including Lake
Pontchartrain. In fact, the storm driven high tide continued to impact the northshore of Lake
Pontchartrain through Friday, August 31. Of note, this storm also produced uncharacteristically
low central pressure given the storm strength, a characteristic that likely also contributed to the
magnitude of the storm surge. On Saturday, September 1, the winds finally died down and the
water slowly started to recede. The slow track that curved north after final landfall coupled with
the intense moisture in the system also meant 48+ hours of intense rainfall throughout Southeast
Louisiana.
Storm impacts where widespread and diverse throughout southeast Louisiana. At the storm’s
peak, over 900,000 homes and nearly half of Louisiana residents were without power. All
Louisiana parishes within the Pontchartrain Basin have been declared eligible for individual and
public assistance by FEMA. Early damages estimates suggest that property damage due to the
storm could run as high as $5 billion. State officials have reported an estimated 59,000 homes
have been damaged by flood, wind, or both, and it is estimated that 26,000 occupied housing
units are located within the flood impacted areas (see Table 1). Thousands of homes in the
Pontchartrain Basin flooded, including 500 homes in Plaquemines Parish and as many as 7,000
residences in St. John the Baptist Parish. Nearly 4,000 residents had to be rescued from surge
induced floodwaters. Three Louisiana residents drowned in floodwaters and two more died due
to circumstances related to wind conditions1. One person remains missing. Of note, all three
drownings occurred in surge inundation zones in the Pontchartrain Basin that are outside the
Federal levee protection system.
While the levee and pumps system around GNO kept the surge out of the protected areas, these
areas still experienced significant rainfall flooding. Finally, as the storm tracked north, it
dumped rainfall on the Northshore watersheds, resulting in widespread riverine flooding,
particularly in Franklinton, La. While the threat subsided without major impacts, rainfall created
two close calls with control structures on the Tangipahoa and Pearl Rivers facing “an imminent
threat of failure” before flood fighting actions relieved pressure on the structures.
This report provides an overview of Isaac’s impacts on the Pontchartrain Basin and is divided
into geographical sections. Sections cover direct storm impacts on the protected areas of GNO,
the Plaquemines Parish Eastbank, the Laplace area, the Slidell-Lacombe area, the MandevilleMadisonville area, and the upland Northshore region while a final section summarizes
environmental impacts of the basin. While Isaac certainly brought considerable damage to
Louisiana, it is also true that Isaac’s impacts are at least an order of magnitude lower than
impacts associated with Hurricane Katrina in 2005.
Figure 1: National Hurricane Center wind swaths for Hurricane Isaac.
1
One resident of Vermillion Parish died after falling from a tree, and another person died in a fire on the Westbank
of Jefferson Parish that firefighters were unable to contain due to wind conditions during the storm. In addition, one
resident of from the Westbank community of Boutte remains missing a full two weeks after the hurricane passed.
Figure 2: Maximum storm surge height, based on ADCIRC simulation. (Data provided by the
LSU Coastal Risk Assessment).
Figure 3: Radar and gauge derived cumulative precipitation for Aug. 29 – Sept 4. along with
selected gauge data. Labels below the points provide the measured rainfall at that gauge. The
figure indicates that nearly all of Southeast Louisiana experienced over 10 inches of rain, with
many areas experiencing up to 20 inches. (Map by author using data from NWS and NHC).
Table 1: Estimated number of homes within the flooded areas of the Pontchartrain Basin.
Figures calculated by overlaying the US Census Housing Tenure (H4) by Block 2010 and the
Hurricane Isaac inundation area as delineated by FEMA . The spatial analysis technique used
assumes that the housing units are uniformly distributed within the census block and does not
consider housing elevation.
Estimated Number of Housing Units in Pontchartrain Basin
Inundated Area by Tenure by Parish
Parish
Ascension
Jefferson
Livingston
Orleans
St. Bernard
St. Charles
St. James
St. John the Baptist
St. Tammany
Tangipahoa
Total
Estimated Total
Occupied Units
4,148
93
2,478
630
0
15
873
3,838
13,566
815
26,456
Estimated Owner
Occupied Units
3,433
17
2,078
488
0
14
726
3,427
9,942
700
20,825
Estimated Renter
Occupied Units
715
76
400
142
0
1
146
412
3,624
115
5,631
Source: Office of Community Development – Disaster Recovery Unit spatial analysis of US Census
Housing Tenure (H4) by block (2010 Annual Census) and the Hurricane Isaac inundation area as
delineated by FEMA. Analysis conducted by Charles Flanagan, PhD.
2) Levee Protected Areas of Greater New Orleans
While Isaac certainly generated a major storm surge within the coastal waterbodies of the
Pontchartrain Basin, it is also true that surge heights were within the design criteria of the GNO
surge protection system. By and large the system mostly performed as expected, with no major
failures in the surge protection system.
Isolated problems with pump stations along with an extended period of intense rainfall lead to
street flooding in lowlying areas, including Gentilly and Lakeview. For unknown reasons,
pumps in Gentilly were unable to keep pace with rainfall, and around mid-day on Wednesday
most of northern Gentilly between Filmore Ave. and Robert E. Lee Blvd. experienced a brief
period of moderate street level flooding. Possible reasons include: 1) too much rain for the
pumps to keep pace, 2) a power outage at the pump stations, or 3) pumping into the London Ave.
canal had to cease due to water levels reaching the safe water level of 5 ft. (It is the author’s
impression that, based on having observed conditions in area through the event, #3 is the most
likely explanation, though this issue deserves further investigation).
Prior to the arrival of Hurricane Isaac, a local investigative journalism website The Lens, ran a
story on “Failed Pumps at Corps pumping station could increase Mid-City flooding risk.” The
article detailed missing parts on two of the pumps at the new constructed floodgate-pumpstation
built at the end of Orleans Avenue canal. These two pumps had previously experienced “some
type of hydraulic failure” and were sent offsite for repair. The 20% reduction (there are ten
pumps total at the station) in capacity to pump from the canal into Lake Pontchartrain implied
that there was also reduced capacity to pump rainwater into the canal, potentially impacted parts
of Mid-City, Lakeview, and City Park. The article quoted USACE Chief of Operations, Chris
Accardo: “If it got to that point, we would have to call the SW&B and tell them to call it off.” It
is not clear if the reduced capacity to pump rainwater from these neighborhoods actually lead to
increased flooding.
Additional problems with the new floodgate/pump stations occurred on the 17th street canal.
Reports indicate that automatic pumps controls failed and workers had to manually control
pumps during severe weather conditions. While the operators where able to start pumps and no
serious flooding occurred, it is important to note that had the storm been stronger, hazardous
weather conditions may have prevented the workers from accessing the site and starting the
pumps.
In general, rainfall was very intense, and isolated street flooding along with wind damage was
observed through the metro area though no major flood impacts have been reported within the
protected areas of GNO.
Figure 4: Street flooding on Wickfield Dr. between Prentiss Ave. and Robert E Lee Blvd. The
floodwaters first started to accumulate on Wednesday, August 29, 2012 around 10:00 A.M. and
remained present for most of that afternoon. (Photo by author).
Figure 5: Missing pump parts on the Orleans Ave canal floodgate/pumpstation near Lake
Pontchartrain. (From http://thelensnola.org/2012/08/27/olreans-missing-pumps/)
3) East Bank of Plaquemines Parish
As shown in Figure 2 above, the East Bank of Plaquemines Parish bore the brunt of Isaac’s storm
surge. Storm surge simulations and watermark observations indicate that water levels reached up
to 14 ft. from Braithwaite to White Ditch, and the USGS gauge at Pointe-A-La-Hache recorded a
peak tide of 13.21 ft. This area is outside of the Federal surge protection system, and its only
structural protection against storm surge is a locally built and maintained back levee that is 7 – 9
ft. in height. At the time, this earthen levee was under construction, and freshly deposited soils
and clays had not been compacted. There was no grass or erosion protection, though parish
officials laid Visqueen on the flood side as an emergency measure to protect the levee from
erosion.
The levee protecting the northern portion of Plaquemines Parish Eastbank was either breached
and/or overtopped, resulting in catastrophic flooding that stretched from Braithwaite to White
Ditch. An article in the Times-Picayune states that parish workers deployed to monitor the levee
observed widespread overtopping at approximately 1:00 A.M.. While the Times-Picayune
article states that no levee breaches were observed, a WWL news broadcast shows footage of
efforts to plug an unintentional levee breach.
Plaquemines parish officials estimate approximately 500 houses flooded, affecting
approximately 1200 residents. Flood waters rushed into these neighborhoods during the early
morning hours on Wednesday and depths reached up to 12 ft. While a mandatory evacuation
order had been issued for the entire Eastbank of Plaquemines parish, a number of residents did
not comply. Nearly 150 people had to be rescued from the floodwaters, and two people died due
to drowning when they could not escape floodwaters that quickly inundated their homes in
Braithwaite. Dewatering operations took approximately 10 days.
Figure 6: Incomplete levee and protective Visqueen near Braithwaite, La. Photo taken on or
around August 25, 2012 (before Isaac’s landfall.) (From
http://www.plaqueminesparish.com/storyDetail.php?ID=47)
Figure 7: Screenshot from a WWL TV broadcast at the site of what was described as an
unintentional, storm induced breach in the levee system. A Louisiana National Guard helicopter
is dropping 3,000 lb sandbags to seal this breach.
Figure 8: Flooded homes in Braithwaite, La. (Via Facebook.)
Figure 9: Aerial imagery showing flooding in Braithwaite, La. (From
http://ngs.woc.noaa.gov/storms/isaac/)
Figure 10: Intentional levee breach to drain floodwaters from populated areas. (From
LaGOSHEP, http://www.flickr.com/photos/lagohsep/7896369570/sizes/m/in/set72157631326524634/)
Figure 11: A September 4 flyover shows that waters still had not drained from Braithwaite one
week after the area was inundated by Isaac’s surge. (Photo by Theryn Henkel for LPBF).
4) Laplace Area
After pushing a tidal surge in Lake Pontchartrain (viz. “filling the lake”), Isaac’s persistent
easterly winds on the day after landfall pushed much of that volume of water (viz. “tilted the
Lake”) toward Lake’s southwest shoreline. The resulting 8 – 10 ft surge then impacted LaPlace,
La., a small suburban community west of GNO. Over 12 subdivisions in and around Laplace
experienced surge induced flooding, and local officials estimate that approximately 7,000 houses
flooded. Parish officials there report 3901 post-storm evacuations, as of Sept. 1. Louisiana
Department of Wildlife and Fisheries (LDWF) Agents rescued 1,435 people and 130 pets from
flooded areas of St. John the Baptist Parish (essentially Laplace). No flood related deaths have
been reported in this region.
Figure 12: Flooded homes near Laplace, La. (Photo from La. GOHSEP,
http://www.flickr.com/photos/lagohsep/7897093630/sizes/c/in/set-72157631328246232/)
5) Slidell-Lacombe Area
Following it’s landfall near Grand Isle on the early morning hours of Wednesday, Hurricane
Isaac followed a northerly trajectory west of GNO and Lake Pontchartrain. As the winds shifted
from easterly to southerly, the storm “tilted the Lake” again, this time toward the northshore.
The resulting 8 – 10 ft surge impacted lowlying areas in and around Slidell and Lacombe, La.
By Thursday morning, nearly 36 hours after Isaac’s initial impacts on Louisiana, the tidal surge
from Lake Pontchartrain pushed into Slidell’s Olde Towne area. On Friday afternoon, the author
observed southerly winds continuing to push water into Slidell’s shoreline. LDWF reports 60
rescues in St. Tammany Parish, most are presumably from the Slidell area.
In addition to storm surge from Lake Pontchartrain, the Northshore also had to contend with
heavy rainfall from Isaac. The rainfall swelled rivers and creeks, while drainage was slowed by
high water levels in the Lake.
Figure 13: National Guard members conducted search and rescue operations in Slidell, La. on
Friday, September 1, a full three days after Isaac began impacting the region. (Photo by Andreas
Moshogianis, LPBF).
Figure 14: Big Branch Bayou, a tributary of Bayou Lacombe, floods a bridge on Highway 190,
near Lacombe, La. (Photo by author).
6) Mandeville-Madisonville Area
The same “tilting the lake” that flooded the Slidell area also pushed the storm surge into
residential areas of the Mandeville-Madisonville area. In Mandeville, the surge reached
approximately 8 ft., while it was as high as 10 ft. in Madisonville. Peak surges also occurred on
Wednesday, as northerly winds “tilted the lake” toward the Northshore. The Mandeville
lakefront flooded, with surge waters reaching 3 - 5 blocks inland. The town of Madisonville,
approximately 1.5 miles from Lake Pontchartrain, experienced widespread street flooding. Due
to widespread adoption of elevation requirements, flood impacts on residences here have been
minimal.
Water mark indicates that the maximum water depth
was approximately 2 ft above the ground here.
Figure 15: Watermark on front porch of a residence in Old Mandeville. Water depths were
approximately 2 ft. at this location 2.5 blocks from the Lakefront. (Photo by Author).
Figure 16: Surge waters remain approximately two blocks from the Old Mandeville lakefront on
Friday afternoon. (Photo by author).
Figure 17: Flooded neighborhood in Madionville, La. Note that most of the homes in the image
are on pilings that place the ground floor above the water level. (Photo from La. National Guard,
http://www.flickr.com/photos/lagohsep/7897600962/sizes/z/in/set-72157631329559582/)
7) Upland Northshore
As Isaac continued to move northward, widespread rainfall occurred throughout the
Pontchartrain Basin, including Northshore watersheds that extend into Mississippi. This heavy
precipitation caused rivers and streams to rise, and flooding impacted numerous homes near
rivers. Possible failure of two control structures necessitated proactive responses and mandatory
evacuations on both ends of the Northshore as a precaution against possibly greater flooding
impacts.
Lake Tangipahoa Dam Failure: Located in Percy Quinn State Park, Lake Tangipahoa is a
manmade lake that feeds the Tangipahoa River. High amounts of rainfall swelled the Lake,
creating a failure risk for the controls structure (earthen dam with concrete spillway). Failure of
the dam would have resulted in widespread flooded along the Tangipahoa River, prompting a
precaution mandatory evacuation order. An estimated 40,000 – 60,000 people could have been
impacted by the flooding. Emergency efforts to redirect some of the water from Lake
Tangipahoa were successful, and the risk of dam failure subsided.
Bogue Chitto Floods Franklinton: As the storm moved inland, extreme rainfall brought a flood
crest down the Bogue Chitto River. Flooding occurred in Franklinton, La. on Friday, August 31
(3 days after landfall). Approximately 130 homes flooded, and an unknown number of residents
had to be rescued.
Pearl River Lock #2: The Bogue Chitto River drains into the Pearl River near Bush, La. As the
flood crest in the Bogue Chitto moved into Pearl River, it met a river already swollen by rainfall.
The extreme waters levels created stress on a navigational control structure, Lock #2, resulting in
erosion around the structure. Citing an “imminent risk” of failure of the structure, authorities
ordered a mandatory evacuation of residences along the Pearl River south of Lock #2. Engineers
subsequently opened a valve, which relieved pressure on the Lock #2. While the Pearl River
remains above flood stage for some time following the passage of the storm, the risk of failure
subsided and the evacuation order was rescinded without major impacts.
8) Environmental Concerns and Impacts
In addition to damaging coastal wetlands, hurricane Isaac’s wind, rain, and surge damaged a
number of industrial locations, resulted in hazardous releases and other environmental concerns.
While environmental impacts are still being assessed, early evidence establishes hazardous
releases and wetland damage.
Releases from Industrial Facilities
Scientists with the Gulf Restoration Network, the Louisiana Bucket Brigade, and the Sierra Club
have been monitoring industrial accidents related to Hurricane Isaac. Their activities include a
review of reports to the Coast Guard’s National Response Center and flyovers on September 2
and 10 of coastal areas in Southeast Louisiana. While assessments are ongoing, their initial
reviews have indentified at least 30 industrial accidents in the Pontchartrain Basin related to
Hurricane Isaac (see Figure 18).
Likely, the most severe incident occurred as a result of the storm surge flooding of the
Stolthaven facility on the Eastbank of Plaquemines Parish. This facility serves as a transfer
station for various materials as they are transferred to and from deep draft. The facility consists
of a number of large storage tanks and a rail yard. Both the storage tanks and rail tankers were
impacted by flooding. Initially, the facility operators reported that 191,000 gallons of various
toxic chemicals have been released. However, they have since qualified this figure as a “worst
case” estimate based on inventory records of everything stored at the facility at the time of
flooding. The definitive figures on the type and amount of chemical released come from a
“Unauthorized Discharge Notification Report” that Stolthaven submitted to Louisiana
Department of Environmental Quality on September 17. It states that 58 gallons of Vivatec,
2,584 gallons of UltraS-4, and 2,883 gallons of Octene were released from the facility.
Structural damage to the facility, including dislodged storage tanks and tanker railcars has been
documented and rescuers in the area reported various chemical smells.
Figure 18: Oil and gas related accidents during Hurricane Isaac reported to the National
Response Center. (From the Louisiana Bucket Brigade).
Figure 19: Stolthaven Transfer Facility in Plaquesmines Parish on September 7, 2012. In
addition to a number of dislocated railcars, the photos shows a large storage tank that had been
moved from its foundation. (Photo by Gulf Restoration Network,
http://www.flickr.com/photos/healthygulf/)
Wetland Loss in the Pontchartrain Basin
Like previous major storms, Hurricane Isaac’s surge likely brought considerable damage to
coastal wetlands within the Pontchartrain Basin. Assessments are ongoing, but aerial imagery
indicates that damage has occurred. Wetland impacts include both direct landloss due the
destructive effects of the surge and waves along with the indirect impacts of increased salinity
due to the surge of saltwater.
The Chandeleur Islands chain is one of the first lines of defense for the Pontchartrain Basin.
Given its location, the Islands endured many hours of wind driven surge and waves.
Assessments are ongoing, but aerial imagery suggests storm induced erosion on the island. It has
also been reported by USGS that sand berms build in front of the islands during the 2010 BP oil
spill were virtually wiped out during the storm, though it is likely that most of that material
remains in the system.
The Breton Landbridge, Biloxi Marsh Landbridge, and the New Orleans East Landbridge all
experienced high energy storm surge and waves impacts that likely resulted in wetland loss.
Even the relatively protected the Maurepas Landbridge, a swamp and marsh habitat on the
western shore of Lake Pontchartrain, experienced surge inundation and a degree of wave
impacts.
Figure 20: Before (left) and after (right) imagery of the Chandeleur showing storm induced
erosion. (Imagery from Google and NOAA, http://ngs.woc.noaa.gov/storms/isaac/)
Figure 21: Apparent wetland loss near Delacroix, La. However, the apparent loss may actually
be temporary inundation due to surge waters not fully receding. (Imagery from Google and
NOAA, http://ngs.woc.noaa.gov/storms/isaac/)